Precious metal alloy



Nov. 21, 1933. WISE 1,935,897

PREcious METAL ALLOY Original Filed Jan. 28, 1929 2 Sheets-Sheet l M AAAWQAWKO O 0 6'0 7O 0 .90 1% 0 10 219 3 u INVENTQR Zebra/70'! Mae BY ala M; ,,M,+M

ATTORNEY Nov. 21, 1933. E M "55 1,935,897

PRECIOUS. METAL ALLOY Original Filed Jan. 28, 1929 2 Sheets-Sheet 2 I 5 mm 0 mvwmm ATTORNEY Patented Nov. 21, 1933 PRECIOUS METAL ALLOY Edmund M. Wise, Jersey City, N. J., assignor to The International Nickel Company, Inc., a corporation of Delaware Original application January 28, 1929, Serial No. 335,481. Divided and this application December 30, 1929. Serial No. 417,335

'7 Claims.

This invention relates to improved alloys of palladium, silver and copper, and to a novel method of heat-treating the same, and more particularly to alloys of the above metals having a high tarnish and corrosion resistance, a high hardness value and other characteristics which render them especially suitable for dental use, and for other advantageous uses such as heat and electrical conductors.

One of the objects of the invention is to provide an improved metal alloy which is white in color, which is resistant to corrosion, and which is both strong and ductile.

Another object is to provide a metal alloy which is particularly adapted for dental, and electrical and heat conductive purposes.

Still another object is to provide an alloy having a high resistance to tarnish, and one having a comparatively high hardness value and tensile strength. 7

The invention also consists in certain new and original features of treatment and combinations of elements hereinafter set forth and claimed.

Although the novel features which are believed to becharacteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its I operation and the manner of its organization may be better undertaken in connection with the accompanying drawings forming a part thereof, in which Fig. 1 is a ternary diagram showing the range in composition of an alloy of palladium, silver and copper in accordance with the present invention; 7

Fig. 2 is a ternary diagram showing the'composition of the alloys divided in accordanu with tarnish resistance;

Fig. 3 is a ternary diagram showing the range of hardenable alloys;

Fig. 4 is a ternary diagram showing the range of alloys which are particularly suitable for dental purposes; and

Fig. 5 is a graphic diagram'showing the effect of heat treatment at various temperatures on the hardness of certain alloys.

In the following description and in the claims elements will be identified by specific names for convenience, but they are intended to be as generic in their application to similar elements as the art will permit.

In accordance with the present invention alloys consisting of palladium, silver and copper have been found to have a resistance to tarnish and 5 corrosion which is dependent upon the relative stood by referring to the following description percentages of the various metals and sometimes upon the heat treatment to which they are subjected. Alloys lying in the area ABC of the diagram of Fig. 2 have been found to be substantially completely resistant to both tarnish and corrosion. Alloys lying in the area DEBA of that diagram are largely resistant to tarnish and corrosion and may be considered within a useful range, while the remaining alloys possess this property to a less degree. The area ABC may be approximately represented analytically by stating that the palladium represents from 35% to 98% and is not less than 35+(6/'7 Cu), the copper content being not less than 1% and the silver constituting substantially the remainder of the alloy, but being not less than 1% of the total. The area of useful alloys represented as DEC on the diagram may be defined analytically as containing from 25% to 98% palladium, the palladium content being not less than 25+(4/11x Cu) the copper being not less than 1% with the silver constituting substantially the remainder of the alloys, but being not less than 1%. The resistance to corrosion and tarnish repre- 30 sented by the curves of Fig. 2 was determined .by subjecting the alloys to moist I-IzS vapor.

The arrangement used consisted of a. desiccator containing a solution of sulphuric acid in water (43% H2SO4 by Wt.). This solution will main- 5 tain 50% relative humidity in the desiccator. A small beaker containing a solution of sodium polysulphide was also placed in the bottom ofthe desiccator which maintains a small concentration oil-12S in the chamber. It should also be noted that the vapor also contains a trace of H2804. The samples preparedby polishing and rubbing with dry alumina were placed on a tray in the desiccator and allowed to remain fifteen hours or longer as indicated. The specimens 5 were then removed and half of the specimen rubbed with alumina to remove any tarnish that might be present. v

The amount of tarnish was then noted and utilized in the formation of the above mentioned diagram. This particular type of a tarnish test is employed because it is the one in common use in the dental trade and is the well known means for classifying alloys-as to their tarnish resistance properties.

It has been found that certain alloys may, by suitable heat treatment, have their hardness and tensile strength materially increased. It has also been found that the heat conductivity and electrical conductivity of the hardenable alloys 0 I coolediromnthistemperature at a rate substancan be increased by a precipitation heat treatment. Referring to Fig. 3, the alloys in which substantial hardening will take place upon heat treatment are represented by a portion of. the area above the dotted line in that figure. It may be approximately represented. analytically by stating that the palladium content varies between the limits of 25% and 62.6% with the copper percentage greater than 5+1.544 10 (Pd-35) and less than 48%, with silver-constituting substantially the remainder of the alloy. The solid curves represent alloys that have been hardened to Rockwell B values of 80-110.

It will be understood that the temperatures for heat treatment maybe varied within considerable limits. Desired resultshave been obtained by heating the alloys to a temperature in excess of 700 C. and then quenching in water.. It was found that in this way the tensile strength and hardness were materially decreased while theductility was increased, so that the quenched alloy could be cold rolled or drawn into wire or sheet form. An increase in tensile strength and hardness was obtained by heating this quenched alloy for a period of time varying from 15 minutes to an hour at to 500 slowly. 4

A considerable increase in tensile strength and hardness will result if after treating at 450 C. the alloy be quenched from this temperature, but the results obtained by slow cooling from that temperature were generally superior.

The increase in hardness on the Rockwell B scale is illustrated for .two representative. alloys in Fig. 5, the alloys having been previously treated at 860 and quenched and subsequently treated at the temperature indicated on the diagram for a period of approximately 30 minutes and quenched. Curve 6 shows the results of heat treatment of an alloy of 40% Pd, 45% Ag, 15% Cu. Curve 7 represents an alloy of 40% Pd, 46% 10% Cu and 4% Ni. It is to be noted that for the alloys shown, a reheating temperature of 450 C. was found to produce the best results.

As an alternative heat treatment by which good results are obtained, the alloy may be annealed at a temperature in excess of 700 C. and

a temp'eratureof from .400" C. and then allowing the alloy to cool tially slower than air cooling: 'The desired rate may be conveniently obtained by permitting the alloy to cool in silocel.

It has been found that an alloy having a composition of 35-55% Pd, 55-5% Ag and the remainder Cu can be given a tensile strength in excess of 100,000 pounds per square inch by suitable heat treatment. As a specific instance, the influence of heat treatment on the tensile strength of an alloy containing substantially 40% palladium, 40% silver and 20% copper may be noted. The ultimate strength of this alloy after quenching from 860 C. was 97,400 lbs. per square inch. After aging for 30 minutes at 450 C. and

'. quenching, the tensile strength increased to 149,000 lbs. per square inch and after aging for 30 minutes at 450 C. and slow cooling, the tensile strength'was increased to 171,000 lbs. per square inch.

, For dental purposes it is very desirable to utilize an alloy which may be hardened to a certain degree and which will be largely resistant to tarnish and corrosion. A suitable range of alloys for this purpose is represented by the area within curve FGHI of Fig. 4. -It is to be noted that this range is located in that area at which for electrical contacts.

may be cooled as by air cooling from a high temsubstantial hardness may be obtained, as shown in Fig. 3, and in which a high degree of tarnish and corrosion resistance is obtained, as shown in Fig. 2.

In addition to the above mentioned features, it has been found that the alloys described are all substantially white with practically a total absence of other colors. This white color renders these-alloys especially'suitable for dental purposes. Furthermore, when properly heat treated, they are of comparatively high electrical conductivity, which in connection with their noncorrosive properties, renders them most suitable The density of the alloys is relatively low and therefore their costs per unit volume is lower than that of many of the gold alloys which they replace.

Alloys in which palladium is present in amounts of less than 25%, possess a'high heat and electrical conductivity and have a melting point much higher than copper-silver'alloys containing no palladium. These alloys may be useful in cases where color and tarnish resistance are of less importance.

Alloys containing 30-90% Pd with copper the principal remainder may be useful for electrical contacts. Two alloys which have been foundparticularly suitable for this purpose comprise Pd Cu 20% and Pd 58% and Cu 42%.

In addition to the three elements, palladium, silver and copper, various other elements may be present in small amounts epending upon the particular requirements of the alloy. For example, the soundness and denseness of the castings may be improved as for instance by the presence of a deoxidizer such as zinc,.silicon, boron, magnesium or calcium or compounds thereof. The deoxidizer should be ametal or metalloid such that the heat of formation of its oxide is substantially in excess of the heat of formation of copper oxide, in order to insure the complete deoxidization of the alloy. Calcium boride in an amount of approximately .2 percent has been found suitable for this purpose. v

Small amounts of other metals may be added to the 'abovealloys without departing from the spirit of the invention. Small amounts of noble metals such as gold, platinum and rhodium may bepresent and bammetals such as nickel or cobalt may be present up to about 4%.

It is obvious that the compositions of the above alloys-and the temperatures employed in heat treatment may vary over a wide range. Certain definite figures have been given by way of example only. Furthermore, the uses of the alloy 139 are not limited to the few instances given. They may be employed in various arts wherein their particular characteristics are of advantage.

In the initial heat treating operation the alloy perature, for example 860" C. and subsequently age hardened as above described. The preferred rate of cooling in this case should be suflicient to prevent the formation of substantial-amounts of a compound composed of palladium and copper in the approximate ratio of their atomic weights. It has also beenfound that slow cooling of alloys within the hardenable range serves to increase their electrical conductivity. Alloys of 40% palladium and 5% to 20% copper, with the remainder silver, are particularly suitable for thm purpose and by slow cooling from a temperature in excess of 700 C. show a marked increase in electrical conductivity.

Although one process for making the improved 150 product has been set forth and described and pointed out in the claims, it is obvious that various changes may be made in-the process or in the separate steps thereof without modifying or changing the essential features and charac teristics of the product produced and that such product remains substantially the same although free from iron and analogous metals impairing the ductility of the alloy.

2. An alloy of palladium, copper and silver, composed of not less than 16% and not more than 48% copper, more than 33% and less than 68% of palladium, and silver to an extent of not less than 1% and constituting substantially the halance, said alloy being substantially free from iron and analogous metals impairing ductility of the up! I 3. An alloy 01 palladium, copperand silver, composed of not less than 16% and notmore than copper, palladium ranging from about 35% toabout 59%, and silver to the extent of not less than 4% and constituting substantially the balance, said alloy having a hardness in excess of Rockwell B, and being substantially free from ironand analogous metals impairing ductility of the alloy. 0 I

4. An alloy containing 40% to 50% palladium, 14% 'to 30% copper and the remainder silver, said alloy being substantially free from iron and analogous metals impairing ductility of the alloy.

5. An alloy of the character set forth in claim ;1, in which the deoxidizer is selected from a group consisting of manganese, zinc, silicon,

boron, magnesium, calcium or compounds there- 6.' An alloy of the character set forth in claim '1 which is adapted to'be hardened by heat treatmentand whichcontains about 25% to about 63% palladium, copper in excess of about 3.5% and less than 48% andsilver not less than 1% and constituting the remainder of the alloy.

7. A non-tarnishing-alloy of the character set forth in claim 1 which containsabout 35% of palladium, not less than about 1% and not more than about 35% of copper, and silver constituting substantially the remainder of the alloy but be-' ing not less than.1% of the total. a 

